1. Field
The present invention relates to thermal printing and, more particularly, to techniques for improving thermal printer output by compensating for the effects of thermal history on thermal print heads.
2. Related Art
Thermal printers typically contain a linear array of heating elements (also referred to herein as “print head elements”) that print on an output medium by, for example, transferring pigment or dye from a donor sheet to the output medium or by activating a color-forming chemistry in the output medium. The array of heating elements is a component of a thermal print head (also referred to herein as a “thermal printing head” or “TPH”) that also includes a support and driving circuitry, as described in more detail below. The output medium is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements, when activated, forms color on the medium passing underneath the print head element, creating a spot having a particular optical density (hereinafter the term “density” refers to “optical density” unless otherwise specified). Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
A thermal print head heating element (also referred to herein as a “heating element” or “print head element”) is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of pigment to the output medium or the formation of color in the receiver. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy provided to the print head element may be varied by, for example, varying the amount of power to the print head element within a particular time interval or by providing power to the print head element for a longer time interval.
In conventional thermal printers, the time during which a digital image is printed is divided into fixed time intervals referred to herein as “print head cycles”. Typically, a single row of pixels (or portions thereof) in the digital image is printed during a single print head cycle. Each print head heating element is typically responsible for printing pixels in a particular column of the digital image. During each print head cycle, an amount of energy is delivered to each print head element that is calculated to raise the temperature of the print head element to a level that will cause the print head element to produce output having the desired density. Varying amounts of energy may be provided to different print head elements based on the varying desired densities to be produced by the print head elements.
One problem with conventional thermal printers results from the fact that their print head elements retain heat after the conclusion of each print head cycle. This retention of heat can be problematic because, in some thermal printers, the amount of energy that is delivered to a particular print head element during a particular print head cycle is typically calculated based on an assumption that the print head element's temperature at the beginning of the print head cycle is a known fixed temperature. Since, in reality, the temperature of the print head element at the beginning of a print head cycle depends on (among other things) the amount of energy delivered to the print head element during previous print head cycles, the actual temperature achieved by the print head element during a print head cycle may differ from the desired temperature, thereby resulting in a higher or lower output density than is desired. Further complications are similarly caused by the fact that the current temperature of a particular print head element is influenced not only by its own previous temperatures—referred to herein as its “thermal history”—but by the ambient (room) temperature and the thermal histories of other print head elements in the print head.
As may be inferred from the discussion above, in some conventional thermal printers, the average temperature of each particular thermal print head element tends to gradually rise during the printing of a digital image due to retention of heat by the print head element and the over-provision of energy to the print head element in light of such heat retention. This gradual temperature increase results in a corresponding gradual increase in density of the output produced by the print head element, which is perceived as increased darkness in the printed image. This phenomenon is referred to herein as “density drift.”
Furthermore, conventional thermal printers typically have difficulty accurately reproducing sharp density gradients between adjacent pixels both across the print head and in the direction of printing. For example, if a print head element is to print a black pixel following a white pixel, the ideally sharp edge between the two pixels will typically be blurred when printed. This problem results from the amount of time that is required to raise the temperature of the print head element to print the black pixel after printing the white pixel. More generally, this characteristic of conventional thermal printers results in less than ideal sharpness when printing images having regions of high density gradient.
The above-mentioned patents and patent applications introduce techniques that obviate many of these problems for thermal printers that print a single color in one pass of the thermal print head. Such methods may also be employed when more than one color is printed in a single pass of a thermal print head onto a thermal imaging member capable of rendering more than one color. Examples of such thermal imaging members, and methods for printing thereon, are described in U.S. Pat. No. 6,801,233, and U.S. patent application Ser. Nos. 11/400,734 and 11/400,735. However, there still remains a need for improved methods for thermal history control when multiple colors are printed in a single pass.
The single-color thermal history control methods of the prior art comprise two distinct models: a thermal model (of the thermal print head) and a “media model” that computes the color density achieved in a thermal imaging member (also known in the art as a “medium”) as a function of a supplied energy (or the inverse of this function). It is straightforward to generalize the prior art thermal model to the case in which multiple colors are printed in a single pass. The parameters of the thermal model may be adjusted to account for the differing printing times and power levels that may be required for different colors, thereby allowing an accurate tracking of the state of the thermal print head (and, in particular, the temperature of the print head elements) while printing. It might be thought that the media model could be carried over to the multicolor case as well, since in its prior art embodiment it requires as input only the current state of the thermal print head, the desired density to be printed, and certain fixed parameters appropriate to that particular color.
However, such a straightforward generalization of the media model may be inadequate for multicolor printing. Problems that may occur include lack of a clean separation between the thermal and the media model, making it difficult to fine tune the thermal history response and/or adapt a thermal history characterization from one thermal imaging member to another; unstable or oscillatory responses to attempts to adjust the thermal model parameters to achieve a desired response; physically unreasonable values being obtained in the thermal model as a result of insufficient flexibility (in technical terms, insufficient degrees of freedom) in the media model; and non-monotonic or ill-defined responses of the thermal history control algorithm over a 3-D color space. Note that when thermal history compensation fails in the multicolor case, not only are distortions in density possible, but distortions in color may occur as well, with objectionable results in a final image. For all these reasons, there is a need for an improved thermal history control algorithm for printing multiple colors on a thermal imaging member with a thermal printer.